The present invention relates to an improvement in a frame-supported pellicle for protection of a photolithographic mask or, more particularly, to an improvement in a frame-supported pellicle consisting of a frame member and a transparent thin membrane of a polymeric resin adhesively bonded to the surface of the frame member with a layer of an adhesive interposed therebetween.
As is known, various kinds of electronic devices such as LSIs, VLSIs, liquid-crystal display units and the like are manufactured by a process involving a photolithographic patterning work on a substrate such as a semiconductor silicon wafer. The photolithographic patterning is conducted by using a photomask bearing a pattern, through which the substrate surface is exposed pattern-wise to ultraviolet light. Along with the trend in recent years toward more and more increased fineness of patterns in the photolithography, it is very important that the photomask used therefor is absolutely free from dust particles deposited thereon in order to ensure accuracy and fidelity of the photolithographically reproduced pattern. In view of the fact that perfect absence of dust particles on a photomask can hardly be achieved even by working in a clean room of the highest cleanness, it is a usual practice to protect the photomask from dust particle deposition by mounting a frame-supported pellicle thereon.
A flame-supported pellicle for photolithography used in the above described purpose consists of a frame member made, usually, from aluminum, stainless steel, polyethylene and the like and a thin membrane of a polymeric material such as nitrocellulose, cellulose acetate and the like having transparency to the light used in the patterning exposure, which is flatly supported under an adequate tension on the frame by adhesively bonding with an adhesive layer therebetween. Since the pellicle membrane is held above the photomask at a certain distance and the exposure light is focused on to the pattern on the photomask, the dust particles, even when they are deposited on the pellicle membrane, cause no particular drawbacks relative to the quality of the photolithographically reproduced pattern.
Various kinds of adhesives are used for adhesively bonding a pellicle membrane to a frame member including those disclosed in U.S. Pat. No. 4,861,402 and Japanese Patent Publication No. 63-27707 such as epoxy resin-based and acrylic resin-based adhesives. Alternatively, as is taught in Japanese Patent Kokai No. 58-219032, a pellicle membrane can be adhesively bonded to a frame member by wetting the surface of the frame member with an organic solvent having good dissolving power for the polymeric material of the membrane and mounting the membrane directly on the thus wetted frame to cause adhesive bonding.
One of the problems in the above described flame-supported pellicle is that, in addition to the requirement for a high adhesive bonding strength between the membrane and the frame member as a matter of course, the adhesive bonding is sometimes not quite reliable in the lapse of time after prolonged use of the pellicle since the adhesive layer is under direct exposure to the ultraviolet light which is detrimental against the polymeric material of the adhesive to cause degradation of the adhesive resin. For example, conventional adhesives based on an epoxy resin or an acrylic resin are not quite satisfactory in this regard and, if not to mention the relatively poor adhesive bonding strength therewith, these adhesives are subject to remarkable photodegradation under irradiation with ultraviolet light so as to become brittle resulting in occurrence of dust particles from the embrittled adhesive resin per se or eventual peeling of the membrane from the frame member. In addition, the above mentioned conventional adhesives based on an epoxy or acrylic resin cannot be used for adhesively bonding a frame member and a pellicle membrane when the membrane is formed from a fluorocarbon group-containing polymer recently proposed as a material of the pellicle membrane by virtue of the very excellent performance thereof as compared with conventional cellulose derivatives because such a fluorine-containing polymer is poorly susceptible to adhesive bonding with these adhesives.